This invention relates to a method for making lead alloy grids for the positive and negative plates of lead acid batteries.
Lead acid batteries are widely used in the automotive industry. The basic lead acid battery cell includes a positive plate, a negative plate, and a sulfuric acid electrolyte. The plates are held parallel and are electrically isolated by a separator. The separator is microporous and allows free movement of charges and ions through it. The space between the plates is filled with the sulfuric acid electrolyte.
The positive plate is made by applying and curing a positive paste to a lead alloy grid. The cured plate contains sulfates, hydroxides, free lead, carbonates and other complex compounds of lead. This complex paste structure is then converted to sulfates by adding acid, a process commonly referred to as xe2x80x9cpickling.xe2x80x9d The pickled plates are then formed, i.e. charged for the first time, by passing electric charge through the cell. This charge is supplied from an external source. The charging reaction is as follows: 
The complex sulfates are converted to lead dioxide, which becomes the positive active mass. When the cell is discharged, the lead dioxide changes to lead sulfate. As the cell is recharged, lead sulfate changes back to lead dioxide, and this process keeps repeating. This cyclic process eventually results in the degradation of the structure of the positive active mass. At that point, the battery cell loses its capacity such that it is no longer useful.
The negative plate construction is similar to that of the positive plate. The negative paste has similar ingredients to the positive paste, but includes an additional material referred to as an expander. The expander provides conditions during forming that result in the formation of a high surface area lead xe2x80x9cspongyxe2x80x9d active mass at the negative collector surface. The negative cure paste starts out as sulfates, etc., and during forming changes to lead structures. The negative active mass thus cycles between lead and lead sulfate.
The lead acid cell system is unique because the electrolyte reacts or participates actively in the energy storage and release process. The function of electrolyte in other systems is generally to conduct charges only. A fully charged lead acid battery cell develops about 2.5 volts. Batteries based on repeated use of active materials are referred to as secondary storage batteries.
Lead acid batteries that require little to no further maintenance throughout the expected life of the battery are of most interest to the automotive industry. This type of battery is typically referred to as a maintenance-free battery. Originally, pure lead was used for making lead acid secondary storage batteries. The pure lead acted both as a current collector and active material. The energy storage quantities, however, were small. As the demand for high energy and power increased, active materials in the form of pastes and grids as collectors were developed. The function of the grid is to collect electric charge and conduct it to the terminal, as well as to provide mechanical support for the active material. The grid material should have high electrical conductivity, good mechanical strength, good resistance to corrosion in sulfuric acid, and good processability.
A considerable amount of effort has been devoted to the type of alloys used for manufacturing positive and negative grids for such maintenance-free, secondary storage batteries. Pure lead met most of the requirements for the grids except mechanical strength. Lead is soft and difficult to process. In the past, the only way pure lead could be hardened or strengthened was by cold working. However, cold working does not produce acceptable mechanical properties. Adding alloying elements to pure lead, and then cold working, hardened the lead and produced desirable mechanical properties. However, electrical conductivity was adversely affected by the alloy additions. Lead based grids were first made by casting techniques using gravity feed and book moulds. Antimony was added to lead to improve its castability. While the antimony addition provided mechanical strength for the lead alloy, it had a negative effect on the life of the battery due to a loss of water and increased self-discharge.
Expanded metal grid technology was next developed, which was less expensive than the cast grids. By this technique, a rolled or wrought alloy strip or a cast strip is slit and expanded using reciprocating dies or the like and then cut into the desired width and height dimension to form the grid. For this process, calcium addition replaced the antimony addition. A continuous cast grid process was also developed using a lead-tin-calcium alloy, which offered improved properties over the lead-antimony cast grids, but was still more expensive as compared to expanded grid processes. Various battery manufacturers currently use all of these processes.
After the grids are fabricated, the active paste is applied to the grids, generally in a continuous process. Thin and perforated paper is then applied on both sides of the plates and the plates are flash dried, which helps in handling so that the plates do not stick together. The pasted grids are heated in a steamer, typically for about 3-4 hours at about 99xc2x0 C. (210xc2x0 F.). Finally, the pasted grids are cured in a process that takes about 3-4 days at a curing temperature of approximately 49xc2x0 C. (120xc2x0 F.). It is during the curing process that the interface between the grid material and the active mass paste develops. The interface layer is essentially a lead oxide layer resulting from corrosion of the grid material. The presence of water is essential for this reaction to proceed. At the end of the curing process, the interface layer is very thin.
The cured plates are assembled into the battery. Each automotive battery consists of six separate cells, each contributing about 2-2.5 volts, and the cells are connected in series inside the battery package. Once the plates are incorporated into the package, sulfuric acid is added to the cells, and the cells are allowed to stand for about an hour for the pickling to occur. The battery is then charged for the first time, referred to as forming. After forming, the acid is removed and fresh acid is added and the battery is sealed.
In use, the interface layer grows during each charging process. Because the battery is discharged and charged repeatedly during use, this interface layer continues to grow thicker over time. Eventually, the interface layer can become thick enough to result in failure of the battery. Due to an increase in under-the-hood temperatures in most automobile applications, however, corrosion at the paste-grid interface has become a greater concern. Batteries are tested for corrosion resistance using the industry-accepted SAE J240 life test, which is conducted at 75xc2x0 C. (167xc2x0 F.). As the temperature increases, the corrosion rate increases even faster, i.e., the rate of reaction is an exponential function of temperature. Consequently, the interface between the active mass (paste) and the collector (grid) corrodes to form and grow the lead oxide layer at a high rate at elevated temperature. This lead oxide layer introduces resistance to the flow of electricity. Resistance is proportional to the thickness of the lead oxide layer and inversely proportional to the surface area of the grid. Thus, as the lead oxide layer thickens, the resistance increases. In some instances, separation occurs between the collector and the active mass, resulting in a loss of electrical contact therebetween, which degrades the battery operation.
By way of further explanation, it is known that the interface layer consists primarily of lead dioxide, which conducts electrons and oxygen ions. The oxygen ions diffuse from the paste side through the interface layer to the grid surface and react with lead. Thus, the interface layer grows in thickness. The volume of oxide formed is roughly 1.4 times larger than the metal volume. In addition to increasing resistance to the flow of electricity, this introduces stresses in the oxide. Depending upon the shape of the underlying metal surface, i.e. convex, flat or concave, the stress state is either tensile or compressive. The lead oxide is a ceramic material, which is very weak in tension as compared to compression. Hence, if the surface is convex, the oxide outer surface is in tension. As the thickness grows, so does this tensile force, eventually resulting in the fracture of this oxide in the direction normal to the metal surface. This condition is acceptable as long as the remaining oxide stays in contact with the metal, such that electrical contact between the grid and the active mass is not totally destroyed. Alternatively, the oxide lifts up from the grid surface due to compressive forces. A complete separation of the active mass from the collector surface (grid) results in a loss of electrical contact, and consequently battery failure. Where the active paste and the interface layer are thin, a flat underlying metal surface works reasonably well. However, this construction suffers disadvantages such as a lack of battery capacity and a higher rate of self-discharge. It is appreciated by those skilled in the materials field that sharp corners, edges, high stress fields, and local chemical differences result in increased corrosion rates, but these conditions cannot always be avoided. Hence, one objective for design and material selection is minimization of corrosion, given that complete elimination is not possible. If corrosion is minimized, i.e., the rate of growth of the oxide layer is decreased, then a lower resistance is maintained for a longer period, and stress-induced failure is delayed.
Numerous research efforts have focused on modifying the alloy composition to remedy the corrosion occurring at the interface between the active mass and the grid. For example, silver additions have been made to reduce corrosion. However, silver additions introduce disadvantages, such as reduced adhesion of the paste to the grid. After years of development focusing upon alloy compositions, the Pb-1.5Sn-0.08Ca alloy composition has become the most widely used composition for lead acid secondary storage batteries, and is considered to be adequate for almost all applications. Corrosion properties, however, are determined both by the chemistry and the processing of the grid material. Despite the efforts that have been made with respect to the material selection, the processing as it affects the interface layer has received little attention. Thus, the method of making the grid also represents a potential research area for reducing corrosion.
Grids have been fabricated by gravity feed book mould processing, by continuous casting using a drum, by the expanded metal technique, and by punching. Gravity feed book mould casting and continuous drum casting use molten metal to make the final grid shape. No mechanical fabrication is used to make or shape the grid. Both the expanded metal technique and punching use a strip of lead alloy metal and either cut and expand the strip or punch the grid from the strip. Punching uses a die to cut a desired shape out of the lead alloy strip and the punched material is physically removed from the strip and then recycled. Because resistance is inversely proportional to the surface area of the grid, less grid material translates to lower surface area and thus higher resistance. The expansion method uses partial cuts, and then stretches the strip normal to the cuts. Thus, no material is physically removed in the expansion method, which is advantageous, but the tools and machinery used for expanding are expensive and intricate, and the structure has large and numerous open areas. The strip used for the expansion method or the punching method may be produced either by conventional rolling techniques or could be cast directly to the desired thickness from molten metal.
Due to the softness of the lead strip, the strip and grid are difficult to handle during grid making and pasting. The minimum thickness to which the strip is cast has been no less than about 0.03 inch and the minimum practical plate thickness with the cured paste and expanded metal grid has been no less than about 0.065 inch. Also, for a given battery capacity, the number of plates per unit volume is limited. Currently, SLI 12V lead acid batteries contain 6 cells connected in series, with each cell consisting of multiple positive and negative plates connected in parallel. However, higher power and voltage batteries require more collector surface area and a greater number of cells, such that there is a need for thinner plates and a method of manufacturing thinner plates, which can be practically implemented. There is further a need for a manufacturing technique for making lead acid battery plate grids that minimizes the effects of corrosion at the interface between the paste and the grid.
The present invention provides a method of making a lead alloy grid for use in a plate for a lead acid battery that enables thin battery plates with a high surface area for minimizing the effects of corrosion. The method of the present invention permits the lead alloy to be formed to a very small thickness while work hardening strengthens the strip for handling by cold working and stress distribution. The work hardening is achieved by creating indentations and protrusions in the thinly cast alloy strip, which in addition to strengthening the strip, provides a desirable surface topography for paste adhesion and interface characteristics. To this end, a lead-tin-calcium alloy, for example Pb-1.5Sn-0.08Ca, is formed such as by direct casting to a thin strip, for example having a thickness in the range of about 0.005-0.015 inch, then the strip is work hardened by forming a plurality of indentations in a surface of the strip. In an exemplary embodiment of the present invention, opposing surfaces of a directly cast strip are embossed to form a pattern on each surface of protrusions and indentations. The work hardening, for example embossing, may further include splitting the strip at a high point of the protrusions and indentations to create perforated protrusions and indentations. The grid may then be used to make a plate by pasting an active mass onto the grid, steaming the pasted grid, and curing the pasted grid to provide a plate having a thickness of about 0.05 inch or less.
The present invention further provides a lead alloy grid for use in a plate of a lead acid battery, the grid comprising a cast lead-tin-calcium alloy having spaced protrusions and indentations on each opposing surfaces of the grid. The protrusions and indentations may be pyramid-shaped, cone-shaped or hemispherical-shaped, for example. The protrusions and indentations may further include a perforation. The grid has a higher surface area than punched or expanded grids, thereby providing a lower resistance.